Aside from defining some details of OffscreenCanvas behavior, this is
intended to have no normative behavior change.

- Standalone OffscreenCanvases do not have automatic expiry.
- transferControlToOffscreen OffscreenCanvases automatically expire when
  their rendering is updated, just like regular canvases except this can
  happen in the worker event loop.

Bonus editorializing, as always:

- Removes the concept of "commit" because this can be more simply
  described as what happens when you read the "bitmap" of a canvas (which
  we already had some text to define). Now the 2 places that expire canvas
  textures just do so directly.
- Moves HTML spec "hooks" into their own section and describe what they
  are.

Fixes 3295 (again)